The present invention relates to the integration of nuclear-radiation imaging, on the one hand, with ultrasound or magnetic resonance imaging, on the other, in order to superimpose the two images, and in order to utilize the structural information of the ultrasound or magnetic resonance for attenuation correction of the nuclear-radiation image.
In essence, two types of medical images may be distinguished:    1. functional body images, such as may be produced by gamma camera, SPECT, and PET scans, which provide physiological information; and    2. structural images, such as may be produced by as x-ray, CT, ultrasound, and (or) MRI scans, which provide anatomic, or structural maps of the body.
A functional image shows the metabolic activity of body tissue, since dead or damaged body tissue absorbs radiopharmacueticals at a different rate from a healthy tissue. For example, a functional image may be used for in-vivo measurements of cardiac rhythm or respiratory rhythm, quantitation of tissue metabolism and blood flow, evaluation of coronary artery disease, quantitation of receptor binding, measurement of brain perfusion, and liver imaging. Additionally, since the uptake rate of radiopharmacueticals is different between healthy tissue and a tumor, and is furthermore different between malignant and benign portions of a tumor, functional images are of importance in tumor localization and volume determination, and especially, localization and volume determination of malignant portions of tumors. However, a functional image may not show structural details.
On the other hand, a structural image reveals almost exclusively structural details—an anatomic map, for example, by distinguishing bones from soft tissue.
Techniques for registering functional and structural images on a same system of coordinates, to produce a combined, or fused image, are known, and are disclosed, for example in the publication to D. A. Weber and M. Ivanovic, “Correlative image registration”, Sem. Nucl. Med., vol. 24 pp. 311-323 (1994), as well as in K. Kneöaurek, M. Ivanovic, J. Machac, and D. A. Weber, “Medical image registration,” Europhysics News (2000) Vol. 31 No. 4, in U.S. Pat. No. 6,212,423, to Krakovitz, dated, Apr. 3, 2001, and entitled Diagnostic hybrid probes, in U.S. Pat. No. 5,672,877, to Liebig, et al., dated Sep. 30, 1997 and entitled, “Coregistration of multi-modality data in a medical imaging system,” in U.S. Pat. No. 6,455,856, to Gagnon, dated, Sep. 24, 2002 and entitled, “Gamma camera gantry and imaging method,” and in commonly owned U.S. Pat. No. 6,567,687, to front et al., issued on May 20, 2003, and entitled, “Method and system for guiding a diagnostic or therapeutic instrument towards a target region inside the patient's body,” all of whose disclosures are incorporated herein by reference.
These techniques may be used, for example, in order to identify features seen on the functional map, based on their anatomic location in the structural map, for example, for the study of cardiac rhythm, or respiratory rhythm.
However, when raw radioactive emission data is superimposed on a structural image, the resultant image fusion may be somewhat erroneous, due to tissue attenuation. Attenuation refers to “the inevitable loss of information in an image due to the interaction of emitted photons with matter, through photon absorption by the photoelectric effect, photon scatter, by the Compton effect, and pair production, involving photons of energies greater than 1.02 Mev. Attenuation decreases the number of photon counts from that which would have been recorded in vacuum. The relative probability of one of these interactions to occur is a function of the incident photon energy and the atomic number (Z) of the interacting matter.
In functional imaging, a radionuclide or a compound labeled with a radionuclide is injected into a subject. The radiolabelled material concentrates in an organ or lesion of interest, and can show a concentration defect. At a prescribed time following injection, the pattern of concentration of the radiolabelled material is imaged by a radioactive emission detector, such as rectilinear scanner, scintillation camera, single-photon emission computed tomography (SPECT) system, or positron emission tomography (PET) system.
The radionuclide imaging procedure requires a means to define the path along which the emitted gamma-ray travels before striking the detector of the imaging system. The path can be a vector path, a line, narrow fan, or a narrow cone as defined by the detector or collimator. In rectilinear scanners, scintillation cameras, and SPECT systems, a collimator (typically made of lead or other high-atomic number material) is interposed between the object and the detector to define the gamma-ray path. In PET, the unique characteristics of positron annihilation radiation are coupled with electronic circuitry to define the vector path. In all cases, the only information obtained when a gamma-ray strikes the detector is the fact that the photon originated somewhere within the object along the vector path projected back from the detector.
For projection imaging systems, a two-dimensional image is formed with the intensity of each picture element, or pixel, proportional to the number of photons striking the detector at that position. In SPECT or PET, the vector paths are determined for multiple projection positions, or views, of the object, and cross-sectional or tomographic images are reconstructed of the object using standard algorithms. Again, the intensity assigned to each vector path is proportional to the number of photons striking the detector originating along the path, and the intensity of each pixel in the reconstructed image is related to these vector path intensities obtained at multiple views.
In radionuclide imaging, it is desirable to obtain absolute values for radionuclide concentrations (or radionuclide uptake) at each point in the image. Attenuation of the emitted photons within the object, before they reach the detector, is a function of the energy of the photons and the exact composition of the material through which the photons pass to reach the detector. Photons emitted deeper within the object have a higher probability of attenuation than those emitted near the surface. In addition, the composition of the material (in terms of effective atomic number Z and electron density) affects the attenuation, with more attenuation if the path passes through high-Z or high-density regions. Thus, in order to calculate absolute uptake or concentration of a radionuclide in a region of an object, it is required that the path length of each type of material or tissue (or effective-Z and electron density path lengths) be known for each vector. Attenuation corrections for emitted photons are made from this knowledge, allowing accurate concentration values to be obtained.
The full clinical potential of radionuclide imaging has been seriously hindered by some important limitations. The spatial resolution and photon statistical limitations of radionuclide imaging frustrate accurate anatomical localization and hinder quantitation of the radionuclide distribution. Photon attenuation has been identified by the American Heart Association and leading nuclear cardiologists as a major deficiency in diagnosis of heart disease with SPECT, and is a major source of error in the measurement of tumor metabolism using radionuclide techniques. Quantitation is further complicated by the need for scatter compensation for imaging with both single-photon and positron-emitting radionuclides.
A number of researchers have shown that many of these limitations can be overcome through use of emission-transmission imaging techniques which combine anatomical (structural) information from transmission images with physiological (functional) information from radionuclide emission images. By correlating the emission and transmission images, the observer can more easily identify and delineate the location of radionuclide uptake. In addition, the quantitative accuracy of measurement of radionuclide uptake can be improved through use of iterative reconstruction methods, which can account for these errors and improve the radionuclide images.
Existing medical imaging instrumentation has been designed for either emission or transmission imaging, but not both, and attempts to perform both compromise one or both of the data sets. In addition, in the early 1990's when much of this work was done, implementation of iterative reconstruction algorithms was too slow to converge and therefore impeded the flow of information in a hospital setting. Virtually all clinical tomographic systems use analytic rather than iterative reconstruction algorithms, which, unlike iterative reconstruction techniques, have the major advantage that the image reconstruction process can occur concurrently with the acquisition of the image data. The efficiency of analytic approaches is compromised by their inability to account for the quantitative errors of photon attenuation, scatter radiation, and spatial resolution losses mentioned above.
The prior art in this field includes several different approaches to localize and quantify the uptake of radionuclides in the human body. One approach uses stereotactic techniques or computer processing methods to correlate functional information from SPECT or PET images with morphologic information from magnetic resonance imaging (MRI) or CT. This technique has the advantage that it can be applied retrospectively without acquiring new image data from the patient. However, these approaches are computationally intensive, require that the patient be scanned separately on two systems, and have only been successful in the head where the skull limits motion of internal anatomical structures.
A second set of prior art describes instrumentation used to detect emission and transmission data using instruments with single or multiple detectors. Several investigators have acquired both the emission and transmission images. with a radionuclide point, line, or sheet used as the transmission source, which is placed on the opposite side of the body from the scintillation camera. This approach has been applied more recently using SPECT. Studies have shown that this technique is capable of producing adequate attenuation maps for attenuation correction to improve quantitation of radionuclide uptake, and that some modest anatomical localization of the radionuclide distribution is also possible.
An alternative approach uses specially-designed instruments for emission-transmission imaging. For example, Kaplan (International Patent Application No. PCT/US90/03722) describes an emission-transmission system in which the emission and transmission data are acquired with the same detector (single or multiple heads). An alternative emission-transmission imaging system (disclosed in SU-1405-819-A) uses x-ray transmission data and two detectors for determining the direction of the photons to improve detection efficiency. However, an exact method of correcting emission data based on transmission data is not described by either Kaplan or in SU-1405-819-A.
Other prior art notes that the map of attenuation coefficients required for the attenuation correction procedure can be obtained from a separate x-ray transmission CT scan of the patient, although a specific method of generating an attenuation map at the photon energy of the radionuclide source is not known. Specific techniques to determine the attenuation map of the patient from single-energy transmission measurement using radionuclide or x-ray sources have been described which are limited to sources emitting monoenergetic (line) spectra rather than broad spectra such as those typically obtained from an x-ray source.
Specific algorithms for correcting beam-hardening artifacts use single-energy x-ray data and dual-energy x-ray data As used herein, the term “single-energy x-ray” describes methods in which an image is generated by integrating the x-ray signal over a single range of photon energies. As used herein, the term “dual-energy x-ray” describes methods in which two images are generated by integrating the signal over two different photon energy ranges. Thus, either “single-energy x-ray” or “dual-energy x-ray” includes methods in which the x-ray source emits an x-ray beam having either a narrow of broad spectrum of energies. Algorithms for correcting beam-hardening artifacts by using basis-material measurements derived from single-energy or dual-energy x-ray data have been presented but without describing how these measurements can be applied to correction of radionuclide data. Especially for single-energy measurements, the correction techniques associated therewith are principally directed at the removal of beam-hardening streaks and nonuniformities, which disturb the qualitative evaluation of images produced with CT.
A key element has been the combination of the emission and transmission data in a reconstruction algorithm which corrects the radionuclide distribution for photon attenuation. Several authors have described analytic algorithms such as filtered backprojection in which the radionuclide data is modified using an attenuation map to correct for attenuation errors. Among their advantages, these analytic algorithms are fast and require only a single step to reconstruct the radionuclide distribution.
However, they are inexact and utilize a uniform attenuation map in which the value of the attenuation coefficient is assumed to be constant across the patient. Other reconstruction algorithms are iterative and use an exact attenuation map and the radionuclide projection data to estimate the radionuclide distribution across the patient. Maximum likelihood estimation is one statistical method that can be used for image reconstruction. A maximum likelihood estimator appropriate for radionuclide tomography based on an iterative expectation maximization algorithm (ML-EM) has been described. The ML-EM algorithm is easy to implement, accounts for the Poisson nature of the photon counting process inherent with radionuclide imaging, and produces better images than filtered backprojection. In addition, ML-EM algorithms can incorporate physical phenomena associated with radionuclide tomography, such as photon attenuation and scatter, detection efficiency, and geometric aspects of the imaging process. Iterative weighted least squares/conjugate gradient (WLS/CG) methods have also been proposed and used for radionuclide tomography. Overall, WLS/CG reconstruction algorithms converge faster than ML-EM procedures, while still incorporating the statistical nature of radionuclide imaging, and permit compensation for photon attenuation and scatter, detection efficiency and geometric response. Iterative algorithms have been successfully used for both SPECT and PET imaging.
The major disadvantage of iterative algorithms is their computational burden, which when introduced, in the early 1990's represented a major obstacle. Iterative algorithms are iterative procedures and are started with an initial image estimate that either corresponds to a constant radionuclide density throughout the image plane to be reconstructed or corresponds to constant density throughout the highly sampled “reconstruction circle” and zero outside this region. This estimate is unlikely to be representative of the actual distribution of radionuclide in a patient, and a large fraction of the total iterations required to generate useful images may be necessary to reveal the real qualitative structure of the radionuclide distribution. Thus, these algorithms often require 30 to 50 iterations to yield visually acceptable images, and possibly several hundred iterations to generate quantitatively accurate reconstructions.
It also is possible to use filtered backprojection to produce initial image estimates for iterative reconstruction algorithms. Filtered backprojection algorithms can operate concurrently with the emission data acquisition, and they are the method currently used for most clinical radionuclide imaging systems due to their efficiency and ability to produce useful images. Unfortunately it is generally not possible to modify filtered backprojection algorithms to accurately account for details of the collimator geometry, or for the effects of scatter, especially in regions where there are large inhomogeneities in these properties, or details of the collimator geometry.
Therefore, this approach can speed up iterative techniques slightly, although the improvement in convergence speed has not been dramatic. Thus, many investigators have pursued various methods of speeding the convergence of ML-EM algorithms or reducing the time required per iteration. Methods include exploiting the symmetry of the imaging system, multigrid approaches, high frequency enhanced filtered iterative reconstruction, expectation maximization search (EMS) algorithms, rescaled gradient procedures, vector-extrapolated maximum likelihood algorithms, and hybrid maximum likelihood/weighted least squares (ML/WLS) algorithms.
However, all iterative reconstruction methods require significantly more computer time than filtered backprojection algorithms to generate useful images. The iterative ML-EM and WLS/CG algorithms mentioned above assume complete sets of radionuclide projection data exists prior to commencement of the reconstruction procedure. The requirement to acquire complete sets of projection data is especially important in radionuclide system because clinical emission imaging systems typically require several minutes to acquire projection data, making iterative reconstruction techniques impractical.
U.S. Pat. No. 5,155,365, to Cann, et al., dated Oct. 13, 1992, and entitled, Emission-transmission imaging system using single energy and dual energy transmission and radionuclide emission data.” whose disclosure is incorporated herein by reference, describes a method of improving radionuclide emission imaging, by correcting emission transmission data for attenuation along calculated path lengths and through calculated basis material. Single or dual energy projector data can be simultaneously obtained with radionuclide emission data to improve localization of radionuclide uptake. Dual energy x-ray projection techniques are used to calculate the path lengths and basis material (bone, tissue, fat). The radionuclide emission data and the transmitted x-ray data are simultaneously obtained using an energy selective photon detector whereby problems of misregistration are overcome. The dual-energy x-ray projection data are utilized to determine material-specific properties and are recombined into an effectively monoenergetic image, eliminating inaccuracies in material property estimation due to beam hardening. Use of a single instrument for simultaneous data collection also reduces technician time and floor space in a hospital.
Additionally, U.S. Pat. No. 5,376,795, to Hasegawa, et al., dated Dec. 27, 1994, and entitled, “Emission-transmission imaging system using single energy and dual energy transmission and radionuclide emission data,” whose disclosure is incorporated herein by reference, describes additional work, in essence, by the same group as that of U.S. Pat. No. 5,155,365, for improving radionuclide emission imaging, by correcting emission-transmission data for attenuation along calculated path lengths and through calculated basis material. X-ray transmission data are used to develop an attenuation map through an object, which is then used in reconstructing an image based on emission data. Specifically, tomographic reconstruction algorithms were used to calculate an attenuation map, which shows the distribution of attenuation coefficients at each point across the volume imaged in the patient. Radiation detection circuitry is provided which has different operating modes in detecting the x-ray and emission photons passing through the object. An iterative process is used to reconstruct the radionuclide distribution using the radionuclide projection data and the attenuation map based on physical characteristics of the object being imaged. Subsets of the complete radionuclide projection data are used to reconstruct image subsets of the radionuclide distribution. The image subsets can be generated concurrently with the acquisition of the radionuclide projection data or following acquisition of all data.
U.S. Pat. No. 5,210,421, to Gullberg, et al., dated May 11, 1993, and entitled, “Simultaneous transmission and emission converging tomography, whose disclosure is incorporated herein by reference discloses a SPECT system which includes three gamma camera heads which are mounted to a gantry for rotation about a subject. The subject is injected with a source of emission radiation, which emission radiation is received by the camera heads. A reconstruction processor reconstructs the emission projection data into a distribution of emission radiation sources in the subject. Transmission radiation from a radiation source passes through the subject and is received by one of the camera heads concurrently with the emission radiation. The transmission radiation data is reconstructed into a three-dimensional CT type image representation of radiation attenuation characteristics of each pixel of the subject. An attenuation correction processor corrects the emission projection data to compensate for attenuation along the path or ray that it traversed. In this manner, an attenuation corrected distribution of emission sources is generated.
Additionally, U.S. Pat. No. 5,338,936, also to Gullberg, et al., dated Aug. 16, 1994, and entitled, “Simultaneous transmission and emission converging tomography,” whose disclosure is incorporated herein by reference, discloses a SPECT system, which includes three gamma camera heads, which are mounted to a gantry for rotation about a subject. The subject is injected with a source of emission radiation, which emission radiation is received by the camera heads. Transmission radiation from a transmission radiation source is truncated to pass through a central portion of the subject but not peripheral portions and is received by one of the camera heads concurrently with the emission data. As the heads and radiation source rotate, the transmitted radiation passes through different parts or none of the peripheral portions at different angular orientations. An ultrasonic range arranger measures an actual periphery of the subject. Attenuation properties of the subject are determined by reconstructing (90″) the transmission data using an iterative approximation technique and the measured actual subject periphery. The actual periphery is used in the reconstruction process to reduce artifacts attributable to radiation truncation and the associated incomplete sampling of the peripheral portions. An emission reconstruction processor reconstructs the emission projection data and attenuation properties into an attenuation corrected distribution of emission radiation sources in the subject.
Furthermore, U.S. Pat. No. 5,559,335, to Zeng and Gullberg, dated Sep. 24, 1996, and entitled, “Rotating and warping projector/backprojector for converging-beam geometries,” a detector head which receives emission radiation projections from the radioisotope with which a subject was injected, and transmission radiation projections from a transmission radiation source disposed opposite the subject from the detector head. A volume memory stores an estimated volume image. For each actually collected image emission data projection set, a projector reprojects a set of projection of the volume image from the image memory along each of the same projection directions as the emission data projections. Each projection is rotated and warped such that rays, which converge with the same angle as the convergence of the collimator on the detector head become parallel. The layers are each convolved with a point response function weighted in accordance with a depth of the corresponding layer in the volume image and corresponding points are summed to create a reprojected projection. A ratio of each collected projection and the reprojected projection is calculated and backprojected into a volume of correction factors. The backprojectioned correction factors for the set of ratios are summed. A memory-updating algorithm multiplies the estimated volume image in the image memory by the sum of the correction factors. This process is repeated iteratively over a plurality of projection directions, each iteration further refining the volume image in the volume image memory.
U.S. Pat. No. 5,672,877, to Liebig, et al., dated Sep. 30, 1997, and entitled, “Coregistration of multi-modality data in a medical imaging system,” whose disclosure is incorporated herein by reference, discloses a method of coregistering medical image data of different modalities. In the method, an emission scan of an object is performed using a nuclear medicine imaging system to acquire single-photon emission computed tomography (SPECT) image data. A transmission scan of the object is performed simultaneously with the emission scan using the same nuclear medicine imaging system in order to acquire nuclear medicine transmission image data. The emission scan is performed using a roving zoom window, while the transmission scan is performed using the full field of view of the detectors. By knowing the position of the zoom windows for each detection angle, the nuclear medicine transmission image data can be coregistered with the SPECT emission image data as a result of the simultaneous scans. Image data of a modality other than SPECT, such as x-ray computed tomography (x-ray CT) data, magnetic resonance imaging (MRI) data, or positron emission tomography (PET) data, is also provided, which it is desired to have coregistered with the SPECT emission data. The nuclear medicine transmission image data is therefore coregistered with the image data of the different modality. As a result, the image data of the different modality becomes coregistered with the SPECT image data.
U.S. Pat. No. 6,310,968, to Hawkins, et al., dated Oct. 30, 2001, and entitled, “Source-assisted attenuation correction for emission computed tomography,” whose disclosure is incorporated herein by reference, discloses a method of ML-EM image reconstruction, for use in connection with a diagnostic imaging apparatus that generates projection data. The method includes collecting projection data, including measured emission projection data. An initial emission map and attenuation map are assumed. The emission map and the attenuation map are iteratively updated. With each iteration, the emission map is recalculated by taking a previous emission map and adjusting it based upon: (i,j,k) the measured emission projection data; (ii) a reprojection of the previous emission map which is carried out with a multi-dimensional projection model; and, (iii) a reprojection of the attenuation map. As well, with each iteration, the attenuation map is recalculated by taking a previous attenuation map and adjusting it based upon: (i,j,k) the measured emission projection data; and, (ii) a reprojection of the previous emission map which is carried out with the multi-dimensional projection model. In a preferred embodiment, with source-assisted reconstruction, the recalculation of the attenuation map is additionally based upon: (iii) measured transmission projection data; and, (iv) a reference or blank data set of measured transmission projection data taken without the subject present in the imaging apparatus.
Additionally, U.S. Pat. No. 6,339,652, also to Hawkins, et al., dated Jan. 15, 2002, and entitled, “Source-assisted attenuation correction for emission computed tomography,” whose disclosure is incorporated herein by reference, discloses a method of ML-EM image reconstruction, for use in connection with a diagnostic imaging apparatus that generates projection data. The method includes collecting projection data, including measured emission projection data and measured transmission projection data. Optionally, the measured transmission projection data is truncated. An initial emission map and attenuation map are assumed. The emission map and the attenuation map are iteratively updated. With each iteration, the emission map is recalculated by taking a previous emission map and adjusting it based upon: (i,j,k) the measured emission projection data; (ii) a reprojection of the previous emission map which is carried out with a multi-dimensional projection model; and, (iii) a reprojection of the attenuation map. As well, with each iteration, the attenuation map is recalculated by taking a previous attenuation map and adjusting it based upon: (i,j,k) the measured emission projection data; (ii) a reprojection of the previous emission map which is carried out with the multi-dimensional projection model; and (iii) measured transmission projection data.
U.S. Pat. No. 6,384,416, to Turkington, et al., et al, dated May 7, 2002, and entitled, “Transmission scanning technique for gamma-camera coincidence imaging,” whose disclosure is incorporated herein by reference, discloses gamma-camera coincidence (GCC) imaging systems and methods, which include a pair of gamma camera imaging heads rotatable about a patient-longitudinal imaging axis. The imaging heads each has a plurality of radiation opaque septa plates extending transversely relative to the imaging axis about which they locate. Adjacent ones of the septa plates are spaced apart along the imaging axis. At least one point source of radiation is thus positionally fixed between a predetermined adjacent pair of the septa plates of one of the imaging heads so as to be concurrently rotatable therewith.
U.S. Pat. No. 6,384,416, hereinabove, further discloses a method of obtaining attenuation map images by gamma-camera coincidence imaging comprising the steps of:
(a) positionally fixing a radiation point source having a radiation energy greater than about 511 KeV between an adjacent pair of plate-shaped radiation opaque septa of one gamma camera imaging head laterally of a patient-longitudinal imaging axis near a diagonal plane extending along the imaging axis between the one imaging head and an oppositely opposed another gamma camera imaging head;
(b) injecting a human or animal subject with a radiopharmaceutical;
(c) conducting a transmission scan by rotating the one gamma camera imaging bead concurrently with the oppositely opposed another gamma camera imaging head about the patient-longitudinal longitudinal imaging axis so that the another gamma camera imaging head acquires transmission scan data therefrom;
(d) conducting an emission coincidence imaging scan of the subject to obtain emission scan data therefrom; and
(e) combining the transmission and emission scan data to obtain attenuation-corrected cross-sectional maps of radioactivity distributions.
U.S. Pat. No. 6,429,434, to Watson, et al., dated Aug. 6, 2002, and entitled, “Transmission attenuation correction method for PET and SPECT,” whose disclosure is incorporated herein by reference, discloses a transmission source, which serves to detect activity from a radiation source for correcting attenuation in either PET mode or SPECT mode. The transmission source includes a detector dedicated to collecting attenuation data in PET mode. A collimated radiation source and a detector are positioned with respect to a tomography device such that only a selected strip of the imaging detector of the tomograph is illuminated such that events unrelated to the attenuation are eliminated. The transmission source can either be a coincidence transmission source or a singles transmission source and includes a collimator in which is disposed a radiation source. An opening is defined by the collimator for exposing a selected portion of the imaging detectors of the tomograph device. Positioned behind the radiation source, relative to the imaging detectors, is the dedicated attenuation detector. In a dual head tomograph device, one transmission source of the present invention is disposed opposite each bank of imaging detectors. The sources and the associated collimators are positioned to the side of each head at a slight angle relative to the respective head. The sources and detectors are fixed relative to the imaging heads. In order to obtain full coverage of the field of view (FOV) in the same manner as for an emission scan, the heads and sources are rotated about the center of the camera. In SPECT mode, the point source is selected to have sufficiently high energy to shine through the patient and the collimators associated with the imaging detector.
U.S. Pat. No. 6,455,856, to Gagnon, dated, Sep. 24, 2002, and entitled, “Gamma camera gantry and imaging method,” whose disclosure is incorporated herein by reference, discloses a gamma camera, which includes first and second detectors. The first detector is located beneath a patient's receiving surface. The second detector is located above the patient's receiving surface. The second detector is movable between operating and retracted positions. The second detector includes a plurality of discrete detector portions, each detector portion having a first radiation sensitive face, which faces an examination region and a second radiation sensitive face. The patient receiving surface generates signals indicative of pressure applied to the patient receiving surface. A movable transmission radiation source provides transmission radiation, interactions between the transmission radiation and the second detector generating Compton scattered radiation at least a portion of which is received by the first detector, coincident radiation being used to generate a transmission attenuation map. The gamma camera also includes an ultrasound device.
U.S. Pat. No. 6,539,103, to Panin, et al., dated Mar. 25, 2003, and entitled, “Method and apparatus for image reconstruction using a knowledge set,” whose disclosure is incorporated herein by reference discloses a method of constructing a non-uniform attenuation map of a subject for use in image reconstruction of SPECT data is provided. It includes collecting a population of a priori transmission images and storing them in an a priori image memory. The transmission images are not of the subject. Next, a cross-correlation matrix is generated from the population of transmission images. The eigenvectors of the cross-correlation matrix are calculated. A set of orthonormal basis vectors is generated from the eigenvectors. A linear combination of the basis vectors is constructed, and coefficients for the basis vectors are determined such that the linear combination thereof defines the non-uniform attenuation map.
A. J. Nygren published in May 1997, in web page www Dot owlnet Dot rice Dot edu/˜elec539/Projects97/cult/node8 dot html a method for an exact attenuation correction, using algebraic reconstruction. Nygren assumed that an attenuation profile of the object being imaged is known. The reconstruction problem is then formulated with pixel weights assigned by a projection operator, which depends on the distance between the pixel and the detector, and on the assumed attenuation profile. Unlike Chang's method, which involves averaging correction factors, this method allows an exact attenuation correction, using algebraic reconstruction methods.
Other publications include, for example, J.-M. Wagner, F. Noo, R. Clackdoyle, G. Bal, and P. Christian, “Attenuation Correction for Rotating Slant-Hole (RSH)SPECT using Exact Rebinning,” in Conference Record of the 2001 IEEE Nuclear Symposium and Medical Imaging Conference, IEEE Catalog Number 0-7803-7324-3 abstract number M8-5, San Diego, USA, November 2002, and F. Noo, R. Clackdoyle, and J.-M. Wagner, “3D Image Reconstruction from Exponential X-ray Projections: a Completeness Condition and an inversion Formula,” in Conference Record of the 2001 IEEE Nuclear Symposium and Medical Imaging Conference, IEEE Catalog Number 0-7803-7324-3, abstract number M9C-4, San Diego, USA, November 2002.
Additionally, M. P. Tornai, et al, published in web page www-mfk Dot hitachi-medical Dot co dot jp/mfk/medix/29—05 dot pdf “Investigation of Large Field-of View Transmission Imaging for Non-uniform-Attenuation Compensation in Cardiac SPECT. Part 1, Phantom Studies. Their results showed that the implementation of Transmission computed Topography (TCT) acquisition, combined with Non-Uniform Attenuation maps (NUA) compensation techniques, which utilized iterative reconstruction algorithms were promising, and yielded suitable compensated images.
In contrast to these, Chang's Attenuation Correction is a simple approach, described in web page 23ku Dot net/˜chibakakugi/kiso/chang Dot html, The Society of Nuclear Medicine Technology in CHINA, which involves averaging correction factors, so as to use a single attenuation correction value for the tissue.
However, the aforementioned patents and publications suffer from a basic drawback. They attempt to arrive at an attenuation correction factor for gamma rays, using data obtained from x-rays, which are in essence, the same kind of radiation. Therefore, these methods are iterative by nature.
Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves in the range of 1 to 5 megahertz, and their echoes. The sound waves travel in the body and are reflected by boundaries between different types of tissues, such as between a fluid and a soft tissue, or between a soft tissue and a bone). The reflected waves are picked up by the ultrasound probe, and the ultrasound instrumentation calculates the distance from the probe to the reflecting boundary, based on the speed of sound in tissue (about 540 m/s) and based on the of travel, which is usually measured in millionths of a second. The distances and intensities of the echoes are displayed on the screen, forming a two-dimensional image.
In a typical ultrasound, millions of pulses and echoes are sent and received each second. The probe can be moved along the surface of the body and angled to obtain various views.
Before the early 1970's ultrasound imaging systems were able to record only the strong echoes arising from the outlines of an organ, but not the low-level echoes of the internal structure. Therefore liver scans, for instance, did not show possible carcinomas or other pathological states. In 1972 a refined imaging mode was introduced called gray-scale display, in which the internal texture of many organs became visible. In gray-scale display, low-level echoes are amplified and recorded together with the higher-level ones, giving many degrees of brightness. In consequence, ultrasound imaging became a useful tool for imaging tumors, for example, in the liver.
A development of recent years is 3D ultrasound imaging, in which, several two-dimensional images are acquired by moving the probes across the body surface or by rotating probes, inserted into body lumens. The two-dimensional scans are then combined by specialized computer software to form 3D images.
Ultrasound probes, are formed of piezoelectric crystals, which produce an electric signal in response to a pressure pulse, and come in many shapes and sizes. The shape of the probe determines its field of view, and the frequency of emitted sound determines the depth of penetration. Generally, the probes are designed to move across the surface of the body, but some probes are designed to be inserted through body lumens, such as the vagina or the rectum, so as to get closer to the organ being examined.
In multiple-element probes, each element has a dedicated electric circuit, so that the beam can be “steered” by changing the timing in which each element sends out a pulse. Additionally, transducer-pulse controls allow the operator to set and change the frequency and duration of the ultrasound pulses, as well as the scan mode of the machine. A probe formed of array transducers has the ability to be steered as well as focused. By sequentially stimulating each element, the beams can be rapidly steered the from left to right, to produce a two-dimensional cross sectional image.
Several modes of operation are known, A-mode, B-mode, Compounded B-mode, and M-mode or Real-Time mode.
The earliest was the A-mode. Originally when a sound pulse was received it was processed to appear as a vertical reflection of a point. It looked like spikes of different heights. The intensity of the returning pulse determined the height of the vertical reflection and the time it took for the impulse to make the round trip determined the space between vertical reflections. This method of display was called A-mode.
Later, the B-mode was introduced, utilizing gray scale. By assigning to the returning sound pulses different shades of darkness, depending on their intensities, the varying shades of gray in the image reflected variations in the texture of internal organs.
A significant step in improving ultrasound imaging was the development of the Compounded B-mode. Here the images produced at each probe position are stored until the probe has completed its traverse across the body. At that point all the individual scan images are integrated and displayed as a cross section of the body.
The M-mode basically takes a B-mode image, and records the images over time, so that images from the same part of the body are observed, at different times, for example, to the heart's motion.
Real-time mode allows for visualizing motion of internal structures in a way that is easy to read and understand. It is actually made up of compound B-mode images in frames of about 30 per second.
It is noteworthy that attenuation correction may also be desired for the ultrasound. For example, U.S. Pat. No. 4,389,893, to Ophir, et al., dated Jun. 28, 1983, and entitled, “Precision ultrasound attenuation measurement,” whose disclosure is incorporated herein by reference, discloses method and apparatus for measuring an ultrasound attenuation characteristic in a region of interest using ultrasound wherein two statistically independent set of values are accumulated as a difference between logarithms of pairs of each signal set, and the attenuation characteristic calculated as a central tendency parameter of each set of values.
Contrast agents may be used in conjunction with ultrasound imaging, for example as taught by U.S. Pat. No. 6,280,704, to Schutt, et al., entitled, “Ultrasonic imaging system utilizing a long-persistence contrast agent,” whose disclosure is incorporated herein by reference.
Magnetic resonance imaging (MRI) is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum, by nuclei having unpaired spins.
The hardware components associated with an MRI imager are:    i. a primary magnet, which produces the Bo field for the imaging procedure;    ii. gradient coils for producing a gradient in Bo;    iii. an RF coil, for producing the B1 magnetic field, necessary to rotate the spins by 90° or 180° and for detecting the NRI signal; and    iv. a computer, for controlling the components of the MRI imager.
Generally, the magnet is a large horizontal bore superconducting magnet, which provides a homogeneous magnetic field in an internal region within the magnet. A patient or object to be imaged is usually positioned in the homogeneous field region located in the central air gap for imaging.
A typical gradient coil system comprises an antihelmholtz type of coil. These are two parallel ring shaped coils, around the z axis. Current in each of the two coils flows in opposite directions creating a magnetic field gradient between the two coils.
The RF coil creates a B1 field, which rotates the net magnetization in a pulse sequence. They may be: 1) transmit and receive coils, 2) receive only coils, and 3) transmit only coils.
In this geometry, for in-vivo MRI, the use of catheters equipped with miniature RF coils for internal imaging of body cavities still requires positioning the patient in a conventional large MRI magnet. This environment can result in deficient images because the various orientations of the RF coil, e.g., in an artery, will not be positioned always colinearly with the RF excitation field.
This problem has been resolved by U.S. Pat. No. 5,572,132, to Pulyer, et al., entitled, “MRI probe for external imaging,” whose disclosure is incorporated herein by reference, wherein an MRI catheter for endoscopical imaging of tissue of the artery wall, rectum, urinal tract, intestine, esophagus, nasal passages, vagina and other biomedical applications is described.
The invention teaches an MRI spectroscopic probe having an external background magnetic field B0 (as opposed to the internal background magnetic filed of the large horizontal bore superconducting magnet.) The probe comprises (i,j,k) a miniature primary magnet having a longitudinal axis and an external surface extending in the axial direction and (ii) a RF coil surrounding and proximal to the surface. The primary magnet is structured and configured to provide a symmetrical, preferably cylindrically shaped, homogeneous field region external to the surface of the magnet. The RF coil receives NMR signals from excited nuclei. For imaging, one or more gradient coils are provided to spatially encode the nuclear spins of nuclei excited by an RF coil, which may be the same coil used for receiving NMR signals or another RF coil.
U.S. Pat. No. 6,315,981 to Unger, entitled, Gas filled microspheres as magnetic resonance imaging contrast agents,” whose disclosure is incorporated herein by reference, describes the use of gas filled microspheres as contrast agents for magnetic resonance imaging (MRI). Unger further describes how gas can be used in combination with polymer compositions and possibly also with paramagnetic, superparamagnetic, and liquid fluorocarbon compounds as MRI contrast agents. It is further shown how the gas stabilized by polymers would function as an effective susceptibility contrast agent to decrease signal intensity on T2 weighted images; and that such systems are particularly effective for use as gastrointestinal MRI contrast media.